V4641 Sagittarii

V4641 Sagittarii is a variable X-ray binary star system in the constellation Sagittarius. It is the source of one of the fastest superluminal jets in the Milky Way galaxy.

In 1999 a violent X-ray outburst revealed it to contain a black hole. At the time, it was considered to be the closest known black hole to Earth, at a distance of approximately 1,600 light-years (490 pc). Later observations showed it to be much farther away, reported in 2001 to be between 7.4 and 12.31 kpc, 6.2 kpc in 2014, and around 6.6 kpc according to its Gaia Data Release 2 parallax.

The star in the binary system is a late B class giant with a mass about three times that of the Sun. It orbits a black hole about twice as massive every 2.8 days. The star is distorted, which causes variations in its brightness as it orbits and rotates. It is also slightly eclipsed by an accretion disc around the black hole. The system usually does not produce a significant amount of x-rays, but undergoes outbursts when the x-ray luminosity increases due to accretion onto the black hole driving superluminal jets.

V4641 Sgr is a source of ultra-high-energy gamma rays.